From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article from 'The Autocar' magazine discussing the principles of friction and lubrication.
| Identifier | ExFiles\Box 76\4\ scan0122 | |
| Date | 12th August 1911 | |
| 290 THE AUTOCAR, August 12th, 1911. Friction and Lubrication. By Robert W. A.{Mr Adams} Brewer, A.M.I.C.E., M.I.M.E., M.I.A.E., F.S.E. ONE of the oldest axioms of the engineering profession is that “heat and work are mutually convertible terms.” Heat, consisting as it does of kinetic molecular energy, can be produced in many ways, one of them being by friction. As far as this matter concerns us, as makers and users of motor cars, it is our object to reduce the amount of work which is transformed into heat by means of friction to its lowest possible value. Friction exists in several forms; there is, for instance, the friction of rest, which is utilised in such an apparatus as the clutch and again between the wheels of the car and the road in order to transmit the driving effort. On the other hand, we have the friction of motion which to some people is the only recognised kind of friction. The word “friction” itself seems to be somewhat of a bogey to those who do not appreciate more than one type of friction, namely, that of motion, and any such appellation as a “friction drive” or “friction gear” conveys to such people at once some loss of power. I do not intend to discuss the friction of rest in this article more than to mention in passing that the value of its coefficient is generally about twice that of the friction of motion. This implies that the effort or pull required to start a body in motion across the surface of another body is twice the pull required afterwards to keep that body slowly moving. When discussing the question of friction the use of the term “coefficient of friction” is employed, and is denoted by the Greek letter µ. Its value signifies the relation of the pull required to keep the body in motion, to the total weight of the body moved; for instance, if a body weighing 10 lbs. be drawn along a smooth surface by a string and the tension in the string necessary were 1 lb. the coefficient of friction would be 0.1. There is a great deal of difference between the laws governing friction of dry as distinct from lubricated surfaces, as in the latter case, if the lubrication be perfect, there is no actual friction of one metal upon the other, as the surfaces are not in actual contact and the work done is really that required to shear the film of oil between the two surfaces. The friction between dry surfaces varies directly as the pressure, is independent of the surface and also of the velocity of rubbing at low velocities, whereas with fluid friction the magnitude is independent of the pressure and varies directly as the wetted surface and also directly as the velocity (V) at low speeds, as V² at moderate speeds, and as V³ at high speeds. The friction of a lubricated surface is a combination of the two frictions, namely, that of liquid and that of the dry surface, and it is this class of friction which we will proceed to discuss. Experiments to find the value of the coefficient of lubricated friction at high speeds give somewhat conflicting results, as the lubricant varies in nature with the temperature, and the well-known experiments of Mr. Beauchamp Tower for the I.M.E. can be consulted upon these points. Briefly, the results may be summed up as follows: that the total friction of a plain journal is constant at all loads, and that µ varies as 1 ÷ the pressure, and that oilbath lubrication is the most satisfactory, the next sponge, and the next syphon. It was also determined that there is a certain position upon the loaded surface where a maximum pressure is reached in a loaded bearing, where the load is vertically downwards; this maximum pressure occurs at any angle to about 45° beyond the ostensible point of maximum pressure upon the bearing. Further to illustrate this let us consider a shaft rotating in a plain bearing, the shaft being steadily loaded, and that, looking upon the end of the shaft, it is rotating in a clockwise direction. Suppose now a horizontal and a vertical line be drawn through the centre of the bearing, the point of minimum oil pressure will occur in the left-hand upper quarter, i.e., 45° from the horizontal, and as we travel in imagination around the bearing between the shaft journal and the brass, the pressure will rise to a certain value just after reaching the top. It will maintain this value at a fairly constant amount until it reaches the bottom, and then a maximum rise of pressure will occur about 45° beyond the bottom, giving a point of maximum pressure at 90° from the point of minimum pressure. Mr. R.{Sir Henry Royce} K.{Mr Kilner} Morcom has shown that at very high speeds the points of maximum and minimum pressure recede from one another until they are in the line of loading. If we consider now the lubrication of a bearing which is always under load, such as a crankshaft | ||